Nuclear Medicine and Biology 41 (2014) 338–342

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Nuclear Medicine and Biology
journal homepage: www.elsevier.com/locate/nucmedbio

Transport mechanisms of hepatic uptake and bile excretion in clinical hepatobiliary
scintigraphy with 99mTc-N-pyridoxyl-5-methyltryptophan☆,☆☆
Masato Kobayashi a,⁎, Takeo Nakanishi b, Kodai Nishi a, Yusuke Higaki c, d, Hiroyuki Okudaira a,
Masahiro Ono a, Takafumi Tsujiuchi a, Asuka Mizutani a, Ryuichi Nishii e, Ikumi Tamai b,
Yasushi Arano d, Keiichi Kawai aaSchool of Health Sciences, College of Medical, Pharmaceutical and Health Sciences, Kanazawa University, Kanazawa, Japan
School of Pharmaceutical Sciences, College of Medical, Pharmaceutical and Health Sciences, Kanazawa University, Kanazawa, Japanc
Graduate School of Medicine, Dentistry and Pharmaceutical Sciences, Okayama University, Okayama, Japand
Graduate School of Pharmaceutical Sciences, Chiba University, Chiba, Japane
Department of Radiology, Faculty of Medicine University of Miyazaki, Miyazaki, Japanba r t i c l e

i n f oArticle history:
Received 20 December 2013
Received in revised form 3 January 2014
Accepted 7 January 2014
Keywords:
Hepatobiliary scintigraphy
Transport mechanism
SLC transporter
ABC transporter
99m
Tc-PMT99m
Tc-pyridoxylaminatesa b s t r a c t
Introduction: In clinical hepatobiliary scintigraphy, 99mTc-N-pyridoxyl-5-methyltryptophan ( 99mTc-PMT)
is an effective radiotracer among the 99mTc-pyridoxylaminates. However, the mechanisms of humanhepatic uptake and bile excretion transport of 99mTc-PMT have not been determined. We thus
investigated the transport mechanisms of human hepatic uptake and bile excretion in hepatobiliary
scintigraphy with 99mTc-PMT.
Methods: Four solute carrier (SLC) transporters involved in hepatic uptake were evaluated using humanembryonic kidney (HEK) and HeLa cells with high expression of SLC transporters (organic aniontransporting polypeptide (OATP)1B1, OATP1B3, OATP2B1, organic anion transporters (OAT)2 and organic
cation transporters (OCT)1) after 5 min of 99mTc-PMT incubation. Metabolic analysis of 99mTc-PMT was
performed using pooled human liver S9. Adenosine triphosphate (ATP)-binding cassette (ABC) transporters for bile excretion were examined using hepatic ABC transporter vesicles human expressing
multiple drug resistance 1 (MDR1), multidrug resistance-associated protein 2 (MRP2), breast cancer
resistance protein or bile salt export pump. 99mTc-PMT was incubated for 1, 3 and 5 min with ATP or
adenosine monophosphate and these vesicles. SPECT scans were performed in normal and Eisai
hyperbilirubinemic (EHBR) model rats, deﬁcient in Mrp2 transporters, without and with verapamil (ratMdr1 and human MDR1 inhibitor) after intravenous injection of 99mTc-PMT.
Results: Uptake of 99mTc-PMT in HEK293/OATP1B1 and HeLa/OATP1B3 was signiﬁcantly higher than that in
HEK293- and HeLa-mock cells. 99mTc-PMT was not metabolized in the human liver S9. In vesicles with highexpression of ABC transporters, uptake of MDR1 or MRP2 was signiﬁcantly higher at all incubation times.
Bile excretion of 99mTc-PMT was also identiﬁed by comparison between normal and EHBR rats with andwithout verapamil on in-vivo imaging.
Conclusions: Human hepatic uptake of 99mTc-PMT was transferred by OATP1B1 and OATP1B3, andexcretion into bile canaliculi via MDR1 and MRP2. 99mTc-PMT hepatobiliary scintigraphy may be a usefulligand as a noninvasive method of visualizing and quantifying hepatobiliary transporter functionality,
which could predict drug pharmacokinetics.
Crown Copyright © 2014 Published by Elsevier Inc. All rights reserved.

1. Introduction☆ Financial support: This study was partly funded by a Grant-in-Aid for ScientiﬁcResearch from Japan Society for the Promotion of Science (23590176, 24601008,
24659558 and 25293260) and the Japanese Society of Nuclear Medicine Technology.
☆☆ Disclosure: There are no conﬂicts of interest.
⁎ Corresponding author at: School of Health Sciences, College of Medical,
Pharmaceutical and Health Sciences Kanazawa University, Kanazawa, Japan, 5-11-80
Kodatsuno, Kanazawa 920-0942, Japan. Tel.: +81 76 265 2500; fax: +81 76 234 4366.
E-mail address: kobayasi@mhs.mp.kanazawa-u.ac.jp (M. Kobayashi).

99m
Tc-N-pyridoxyl-5-methyltryptophan ( 99mTc-PMT) is an effective radiotracer among the 99mTc-pyridoxylaminates, and is used forthe hepatobiliary scintigraphy to diagnose disease of the hepatobiliary
function and system in Japan [1,2]. Fig. 1 shows some of the humantransporters involved in biliary formation. In human liver, organic
anion transporting polypeptide (OATP)1B1, OATP1B3, OATP2B1,
organic anion transporters (OAT)2 and organic cation transporters(OCT)1 isoforms are mainly expressed at the sinusoidal membrane assolute carrier (SLC) transporters [3,4]. On the other hand, Adenosine

0969-8051/$ – see front matter. Crown Copyright © 2014 Published by Elsevier Inc. All rights reserved.
http://dx.doi.org/10.1016/j.nucmedbio.2014.01.004

M. Kobayashi et al. / Nuclear Medicine and Biology 41 (2014) 338–342

a

b

Blood

Hepatocyte

c

Blood

Hepatocyte

MDR1

MDR1

OATP2B1
MRP2

OATP1B3

Bile
duct

MRP2
OATP1B3

BCRP

OATP2B1

Bile
duct

MRP2
OATP1B3

BCRP
OAT2

OAT2
BSEP

Hepatocyte

OATP1B1

MDR1

OATP2B1

OCT1

Blood

OATP1B1OATP1B1

339

Bile
duct

BCRP
OAT2

BSEP
OCT1

BSEP
OCT1

Fig. 1. Some of the human transporters involved in biliary formation (a), EHBR model rats (deﬁcient in rat Mrp2 transporters) (b), EHBR model rats with verapamil (competitive rat
Mdr1/human MDR1 inhibitor) (c). In the sinusoidal membrane, organic anion transporting polypeptides (OATP)1B1, OATP1B3, OATP2B1, organic anion transporter (OAT)2 andorganic cation transporter (OCT)1 isoforms are mainly expressed as several solute carrier (SLC) transporters. In the bile canalicular membrane, multidrug resistance protein 1 (MDR1),
multidrug resistance-associated protein 2 (MRP2), breast cancer resistance protein (BCRP) and bile salt export pump (BSEP) are expressed as ATP-binding cassette (ABC) transporters.

triphosphate (ATP)-binding cassette (ABC) transporters are mainly
located on the bile canalicular membrane in the liver. The types of ABC
transporter include multidrug resistance protein 1 (MDR1), multidrug
resistance-associated protein 2 (MRP2), breast cancer resistance
protein (BCRP) and bile salt export pump (BSEP) [3,4].
There are two groups of imaging agents in hepatobiliary scintigraphy [5]. Among 99mTc-iminodiacetic acid (IDA) analogs, 99mTcdiisopropyl phenylcarbamoyl IDA ( 99mTc-disofenin) [6] has highhepatic extraction via human OATPs [5,7] and rapid bile canalicular
excretion via human MRP1 and MRP2 [8]. In addition, 99mTc-3-bromo2, 4, 6-trimethyl IDA (99mTc-mebrofenin) [9] is taken up into theliver by human OATP1B1 [10–12] and OATP1B3 [10,11], and is thenexcreted by rat Mrp2/human MRP2 [13], but not human OATP2B1[10], organic cation transporter (OCT)1 [12] or natrium taurocholate
cotransporting polypeptide (NTCP) [10–13]. Among the 99mTc-pyridoxylaminates, 99mTc-pyridoxylideneglutamate ( 99mTc-PG) [14]and 99mTc-pyridoxylideneisoleusine (99mTc-PI) [15] have been reported
as useful in hepatobiliary scintigraphy. However, the transportmechanisms of hepatic uptake and bile excretion of 99mTc-PMT havenot been determined.
The purpose of this study is to clarify the mechanisms of hepatic
uptake of 99mTc-PMT, a kind of 99mTc-pyridoxylaminates, using
human embryonic kidney 293 (HEK293) and HeLa cells expressing
SLC transporters, and the mechanisms of bile excretion using thehuman hepatic ABC transporter vesicles and in-vivo imaging.

2.2. Uptake experiments with HEK-293 and HeLa cells, and vesicles
99m

Tc-PMT was purchased from Japan Medi-Physics Co. (Chiba,
Japan). We ﬁrstly conﬁrmed expression levels of SLC transporters inthe HEK-293 and HeLa cells expressing SLC transporters using 3Hestrone-3-sulfate ( 3H-E3S) (American Radiolabeled Chemicals Inc., St
Louis, MO). One day before the uptake experiments, HEK-293 andHeLa cells expressing a SLC transporter were prepared at 4 × 10 5
cells/well in 12-well plastic plates (n = 4). Cells were pre-incubatedfor 10 min using modiﬁed Hank’s balanced salt solution. Each cell was
incubated by 99mTc-PMT (37 kBq) for 5 min and removed from thetissue culture by 0.25% trypsin-EDTA solution (Sigma-Aldrich, St
Louis, MO, USA). Then, radioactivity of the cells was measured using a
gamma counter. For protein assay of the cells, cellular protein
content was measured with a BCA protein assay kit (Thermo
Fisher Scientiﬁc Inc., Waltham, MA, USA) using BSA as a standard.
Uptake of 99mTc-PMT in HEK293/OATP1B1, HEK293/OATP2B1,
HEK293/OAT2, HEK293/OCT1 and HeLa/OATP1B3 cells was comparedwith that in mock of HEK293 and HeLa cells.
Bile excretion was measured using ABC transporter vesicles
transfected with MDR1, MRP2, BCRP or BSEP (n = 4). The
vesicles were pre-incubated for 10 min using reaction buffer in thekit. 99mTc-PMT (37 kBq) was incubated with each vesicle solution for
1, 3 and 5 min with ATP or AMP solution, and radioactivity wasmeasured using a gamma counter. Uptake of 99mTc-PMT in the ATPsolution was compared with that in AMP solution.

2. Material and methods

2.3. TLC metabolite analysis of 99mTc-PMT in pooled human liver S92.1. Cells and vesicles

99m
Tc-PMT (370 kBq) was added to each of 3 samples including
pooled human liver S9 (BD Biosciences, Woburn, MA) in pure water,
0.167 mM ethylenediaminetetraacetic acid/0.33 M NaK phosphoricacid buffer (pH7.4) with or without a nicotinamide adenine dinucleotide phosphate (NADPH)-generating system. Samples withoutpooled human liver S9 and NADPH were used as controls. 99mTcO4−
extracted from the 99Mo- 99mTc generator was added to control
samples. These samples were stirred and incubated for 5, 10, 30 or
60 min in a water bath at 37 °C and perchloric acid was added to stop
the reaction. Metabolite analysis was performed using reverse phasepartition thin layer chromatography (TLC), with each sample spottedat 1 μl and 2-butanone/metanol/2 M KCl solution (10/9/1) used as
developing solvent. Lengths of the thin layers (8.0 cm) were cut into
1.0 cm sections, and radioactivity was measured using a gamma
counter. Relative abundance was calculated using averaged radioactive count for each thin layer divided by a maximum radioactive countin all thin layers. The identity of each metabolite was conﬁrmed basedon the Rf values of the TLC spots.

For SLC transporters, HEK293 cells expressing human OATP1B1,
OATP2B1, OAT2, OCT1 and plasmid vector alone were prepared as
described previously [16]. Brieﬂy, HEK293 and HeLa cells weretransfected with the respective plasmid DNA, and were then selectedwith the appropriate antibiotics; cells were designed HEK293/
OATP1B1, HEK293/OATP2B1, HEK293/OAT2, HEK293/OCT1 andmock cells, and HeLa/OATP1B3 and mock cells, respectively. All celllines were grown in Dulbecco’s modiﬁed eagle medium (Wako Pure
Chemical Industries Ltd., Osaka, Japan) supplemented with 10% (v/v)
fetal bovine serum (Life Technologies, Carlsbad, CA), 100 U/ml
penicillin and 100 μg/ml streptomycin at 37 °C and 5% CO2.
For ABC transporters, we used vesicles (GenoMembrane Inc.,
Kanagawa, Japan) with high expression of human MDR1, MRP2, BCRP
and BSEP. Experimental kits were also purchased from the GenoMenbrane Inc., (Kanagawa, Japan) and were used for experiments witheach ABC transporter.

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M. Kobayashi et al. / Nuclear Medicine and Biology 41 (2014) 338–342

2.4. Animal experiments

2.5. Statistical analysis
Statistical signiﬁcance of differences was determined with Student
t test or analysis of variance (ANOVA) and Dunnett’s test, which wereused as multiplex analysis with a population mean value, using JMP
software version 9 (SAS Institute Inc., Cary, North Carolina). A P valueof less than 0.01 or 0.05 was considered statistically signiﬁcant.
3. Results
Fig. 2 shows the uptake of 99mTc-PMT in HEK293 and HeLa cells
expressing SLC transporters. All HEK293 and HeLa cells expressing SLC
transporter showed signiﬁcantly higher activity using 3H-E3S whencompared with HEK293- and HeLa-mock cells (P b 0.01). Uptake of99m
Tc-PMT in HEK293/OATP1B1 and HeLa/OATP1B3 was signiﬁcantly
higher than that in HEK293- and HeLa-mock cells (control) (P b 0.05
and P b 0.01, respectively), while there were no differences in uptakeof 99mTc-PMT in HEK293 cells with OATP2B1, OAT2 and OCT1.
Fig. 3 shows the metabolic analysis of 99mTc-PMT in pooled humanliver S9. Rf values of 99mTc-PMT and 99mTcO4− were 0.88 and 0.13,
respectively. Values of 99mTc-PMT in metabolic analysis were 0.88 at 5,
10, 30 and 60 min after metabolic analysis of pooled human liver S9.
Fig. 4 shows the uptake of 99mTc-PMT with ATP or AMP solution
in the vesicles with hepatic ABC transporter (MDR1, MRP2, BCRP,
or BSEP) at 1, 3 and 5 min of incubation. Comparing the uptakeof 99mTc-PMT in the ATP and AMP solution, uptake of ATP solution in
the vesicles with high expression of MDR1 or MRP2 was signiﬁcantly
higher than that with AMP solution at all incubation times. On theother hand, there were no signiﬁcant differences in BCRP or BSEP.
Time activity curve of the liver using dynamic imaging withnormal and EHBR rats is shown in Fig. 5. In the liver of normal rats,
radioactive counts reached a peak at about 2 min and decreased
rapidly. On the other hand, in the liver of EHBR rats, peak time andFig. 2. Uptake of 99mTc-PMT in human HEK293 and HeLa cells expressing SLC
transporter. Uptake in HEK293/OATP1B1 and HeLa/OATP1B3 was signiﬁcantly higher
than that in HEK293- and HeLa-mock cells. ** P b 0.05 and * P b 0.01 vs. HEK293-mockcells or HeLa-mock cells by the Student t test.

washout were delayed when compared with normal rats. In addition,
radioactive counts in EHBR rats with verapamil were signiﬁcantly
higher than those in EHBR rats without verapamil. We were able toidentify these results in average images of 99mTc-PMT at 6–10 min innormal rats (Fig. 5b), EHBR rats (Fig. 5c) and EHBR rats withverapamil (Fig. 5d).
4. Discussion
In this study, we assessed the human hepatic uptake and bile
efﬂux mechanisms of hepatobiliary scintigraphy with 99mTc-PMT, arepresentative 99mTc-pyridoxylaminate radiotracer. 99mTc-PMT was
mainly incorporated by OATP1B1 and OATP1B3 into the liver, as
uptake of 99mTc-PMT in human HEK293/OATP1B1 and HeLa/OATP1B3
cells was signiﬁcantly higher than that in mock cells (Fig. 2). The
chemical structure of tryptophan in the 99mTc-PMT has a carboxylic
acid group and an anionic charge. In general, OATPs transport anionicsystem compound with high molecular weight and OATs comparatively transport the compound with low molecular weight [3,4]. We
expected that uptake of 99mTc-PMT involves OATPs or OATs before we
performed the experiment and we found that 99mTc-PMT was onlytransported by OATPs, as the molecular weight of 99mTc-PMT is relatively high (N 800 Da).
Hepatic uptake of 99mTc-PMT has the potential to yield metabolicchanges. We performed TLC analysis using pooled human liver S9 to
conﬁrm the metabolism of 99mTc-PMT. 99mTc-PMT was not metabolized
in the human liver S9, as the Rf value of 99mTc-PMT (0.88) was the sameas that in control samples with 99mTc-PMT (0.88) and different fromthat of 99mTcO4− (0.13) (Fig. 3). Therefore, we assume that 99mTc-PMT is
not metabolized in clinical hepatobiliary scintigraphy.

1.00

Relative abundanceAnimal experiments were approved by the Chiba University
Committee for the Care and Use of Laboratory Animals, and werecarried out in accordance with the guidelines for the Care and Use ofLaboratory Animals. Three six-week-old male SD normal rats (n = 3)
and Eisai hyperbilirubinemic (EHBR) model rats (Fig. 1b), which haveMrp2 deﬁciency, (n = 3) (280–300 g) were purchased from Sankyo
Labo Service Corporation, Inc. (Tokyo, Japan). Rats were fasted with nofood overnight with water supplied ad libitum before experiments.
During experiments, rats were anesthetized with 1.2%–1.5% isoﬂurane(Abbot Laboratories, Green Oaks, IL). A PE-50 catheter was inserted into
the femoral vein for 99mTc-PMT administration. Imaging data wereacquired using the dynamic planar protocol with a small animal SPECT/CT scanner FX-3200 (Gamma Medica-Ideas Inc., Northridge, CA) [17].
Rats were placed in the supine position on the scanner bed, and limbs
were ﬁxed using surgical tape. The orientation of renal position wasdetermined using a laser beam and CT imaging on the scanner. Aftersetting the radius of rotation to be maximally close to the rats, dynamic
scans (30 s × 4 frames, 60 s × 8 frames and 120 s × 10 frames) with asingle pinhole collimator were initiated with an intravenous bolus
administration of about 74 MBq 99mTc-PMT using a micro-injection
pump CFX1010 (ISIS, Osaka, Japan) for 15–20 s. On the next day, thesame EHBR rats were used for experiments using administration of
0.5 mM verapamil, inhibitor of rat Mdr 1 and human MDR1 (Fig. 1c),
immediately before a 74 MBq 99mTc-PMT injection. Voxel sizes were0.53 × 0.53 × 0.37 mm with a single pinhole collimator. These image
data were analyzed using amide’s a medical image data examiner, opensource analyzing software tool. Five regions of interest were placed overthe liver in 99mTc-PMT images with reference to CT images. Hepatic
uptakes of 99mTc-PMT were compared among normal rats, EHBR rats
and EHBR rats with verapamil.

0.80
0.60

99mTcO 4

99mTc-PMT

5 min
30 min10 min60 min

0.40
0.20
0.00

0.13

0.25

0.38

0.50

0.63

0.75

0.88

1.00

Rf values
Fig. 3. Metabolic analysis of 99mTc-PMT in pooled human liver S9. Rf values of 99mTcPMT and 99mTcO4− were 0.88 and 0.13, respectively. Values for 99mTc-PMT on metabolic
analysis were 0.88 at 5, 10, 30 and 60 min in pooled human liver S9.

M. Kobayashi et al. / Nuclear Medicine and Biology 41 (2014) 338–342

341

Fig. 4. Uptake of 99mTc-PMT with ATP (■)or AMP (●) solution in human hepatic ABC transporter vesicles expressing MDR1 (a), MRP2 (b), BCRP (c) or BSEP (d). In the case of vesicles
with high expression of MDR1 and MRP2, uptake of 99mTc-PMT with ATP solution increased signiﬁcantly as compared to that with AMP solution at 1, 3 and 5 min. ** P b 0.05 and* P b 0.01 vs. AMP solution by the Student t test.

aRadioactive counts (cps)8* * * ** * *

7
6

*

5

**

*

*

*

*
*

*
**

4

*

*

*

*

*

*

*

*

*

*

*
*

*

*

3

*

*

2
1
0
0

5

10

15

20

25

30

Time (min)

b

c

d

Fig. 5. Time activity curve of hepatic radioactivity in dynamic imaging with normal rats(●), EHBR rats without verapamil (▲) and EBHR rats with verapamil (■). In the liver(a), peak time and washout in EHBR rats were delayed when compared with thenormal rats. Radioactive counts in EHBR rats with verapamil were signiﬁcantly higher
than those in EHBR rats without verapamil. This trend was visible in the average images
for 99mTc-PMT from 6 to 10 min in normal rats (b), in EHBR rats (c) and in EHBR rats
with verapamil (d). * P b 0.01 vs. normal rats. ✝ P b 0.05 vs. EHBR rats by Dunnett’s test
after ANOVA.

99m
Tc-PMT is secreted into the gallbladder with bile in humans.
Using vesicles with high expression of ABC transporter, we foundthat 99mTc-PMT was excreted via MDR1 and MRP2 because 99mTcPMT was incorporated into the vesicles with high expression ofhuman MDR1 and MRP2 using ATP dependent transport (Fig. 4).
Among the IDA group, 99mTc-mebrofenin has commonly been
applied to hepatobiliary scintigraphy worldwide [9]. 99mTc-mebrofenin is transferred by human OATP1B1 [10–12] and OATP1B3 [10,11]
into the liver, and by human MRP2 [13] into the bile. 99mTc-disofenin[6] has also been proposed for hepatological scintigraphy and has
been reported to be taken up by OATP into the liver, [5,7] and excretedby human MRP1 and MRP2 into the bile [8]. From the results of these
IDA groups, we expected that human OATP and MRP may be also
involved in hepatic uptake and efﬂux of 99mTc-pyridoxylaminates,
including 99mTc-PMT. However, these IDA groups were not evaluatedother human ABC transporters except MRPs.
Among other imaging agents, 99mTc-hexakis-2-methoxyisobutylisonitrile ( 99mTc-MIBI) has been used clinically for myocardial
perfusion imaging, and it is a substrate for human MDR1 [8,13,18],
MRP1 [8,19] and MRP2 [8] in hepatocytes and/or cancer cells.
Recently, new agents have been developed for hepatobiliary scintigraphy with positron emission tomography (PET) [20,21]. These agentsare also transported by human OATPs and MRP2. However, PET lacks
versatility because a cyclotron is required to produce radioisotopes.
Among the agents for magnetic resonance imaging for measurementof hepatic function, gadolinium ethoxybenzyl diethylenetriamine
pentaacetic acid (Gd-EOB-DTPA) is transferred via human OATP1B1,
OATP1B3 and NTCP, but not OATP2B1, into the liver [22], and is
excreted via rat Mrp2 into the bile [23]. Human ABC transporters havenot been assessed with Gd-EOB-DTPA. In addition, we are unable touse Gd-EOB-DTPA for patients with Gd allergic reactions, serious
kidney damage or hyperbilirubinemia. Therefore, 99mTc hepatobiliary
scintigraphy is the best method to assess SLC and ABC transporters inhepatocytes. Among all imaging agents, metabolic analysis has

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M. Kobayashi et al. / Nuclear Medicine and Biology 41 (2014) 338–342

scarcely been performed. If these agents are metabolized in the liver,
metabolic substances will be involved in the excretion of agents fromhepatocytes into the bile on in-vivo imaging. On the other hand, as
99m
Tc-PMT is not metabolized and unchanged 99mTc-PMT is excreted
into the bile, 99mTc-PMT is suitable for measuring accurate hepatobiliary transporter activity in clinical hepatobiliary scintigraphy.
As shown in Fig. 4, it was conﬁrmed that 99mTc-PMT is transferred
into bile via rat Mrp2 in a comparison between normal and EHBR rats
(Fig. 1b), which is deﬁcient in Mrp2 [24], as washout of 99mTc-PMT
from liver to bile was rapid in normal rats, but was slow in EHBR rats.
Furthermore, washout was delayed in EHBR rats with verapamil (ratMdr1 and human MDR1 inhibitor) (Fig. 1c). The distribution changesof 99mTc-PMT in EHBR rats without verapamil to normal rats werelarger than those in EHBR rats with verapamil to EHBR rats without
verapamil because the effect of MRP2 may be larger than that of MDR1
on the excretion into bile canaliculi of 99mTc-PMT. Therefore, wereconﬁrmed the 99mTc-PMT was excreted via MDR1 and MRP2 on
in-vivo imaging.
Numerous chemotherapeutic drugs for tumor treatment has
been taken up into the liver via OATPs, metabolized and excreted
via MDR1 and/or MRP2 into the bile [4] and act as both substrates
and inhibitors of hepatic transporters such that unexpected andunwanted interactions are frequency observed. Hepatobiliary
scintigraphy may be able to measure the hepatobiliary transporter
activity on pharmacokinetics of the chemotherapeutic drugs [7]and diagnose hepatic diseases associated with a particulartransporter dysfunction. It is important to measure hepatobiliarytransporter activity before administration of chemotherapeutic
drugs because the drugs cause multiple side effects for patients.
Therefore, 99mTc-PMT imaging will be useful as a noninvasive
method to visualize and quantify altered hepatobiliary transporter
activity and to predict drug pharmacokinetics.
5. Conclusion
This study showed that the 99mTc-PMT is mainly accumulated inhepatocytes by OATP1B1 and OATP1B3, and unchanged 99mTc-PMT
is excreted into bile canaliculi via MDR1 and MRP2 in clinicalhepatobiliary scintigraphy. 99mTc-PMT hepatobiliary scintigraphy
may be useful as a noninvasive method of visualizing andquantifying hepatobiliary transporter functionality, which could
predict drug pharmacokinetics.
Acknowledgment
The author would like to thank the laboratory staff of Kanazawa
and Chiba University. This study was partly funded by a Grant-in-Aid
for Scientiﬁc Research from Japan Society for the Promotion of Science(23590176, 24601008, 24659558 and 25293260) and the JapaneseSociety of Nuclear Medicine Technology.
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